Django-systemjs

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* Fixes in the documentation
* Added the ``--minify`` option for bundling

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* Added a staticifles finder: ``systemjs.finders.SystemFinder``. Thanks to
https://github.com/wkevina for the report that lead to this.

* Added proper documentation on readthedocs.org

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Fixes loading of the configuration options in tests.

In previous releases, using ``django.test.utils.override_settings`` in certain
ways happened before the django-systemjs settings were appended. This was
probably only the case for Django < 1.9.

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Small feature release with improvements:

* Added ``systemjs.storage.SystemJSManifestStaticFilesStorage``
staticfiles storage backend. **It is required if you want to use the
``django.contrib.staticfiles.storage.ManifestStaticFilesStorage`` with
django-systemjs.**

django-systemjs runs the post-process hook, which in turn calls
``save_manifest``. The Django shipped version then deletes the old manifest,
but we don't know anything about the staticfiles in django-systemjs. This
storage backend makes sure to not delete the staticfiles manifest, it only
adds to it.

* Better treatment of ``sourceMap`` comments.

JSPM creates the sourcemap and adds a ``// sourceMap=...`` comment to the
generated bundle. django-systemjs added the ``System.import(...)`` statement
to this bundle, causing the sourcemap comment to not be the last line of the
file. This is fixed in this release.

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Too soon on 1.0.1, missed another extension append case.

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Bugfix related to ``DEFAULT_JS_EXTENSIONS``: when bundling a file that has the
extension, it would be added again, leading to duplicate extensions.